Muscles of the Thumb

1. The movements of the thumb are performed by eight muscles.
2. Four of the muscles are situated in the forearm and four in thenar eminence. ^[1]

Of the four muscles in the forearm three are extensors, one for each thumb-joint. The action, however, is not entirely isolated. One of these extensors (longus pollicis) extends the thumb tip, but if the movement be continued it acts also upon the other thumb-joint. This extensor is attached to the bone differently from the similar tendon in the other fingers. Accordingly, the impossibility of extending the nail-joint while flexing the other joints does not hold for the thumb. As in the case of all the muscles thus far considered, the muscles of the thumb show clearly the integrative action of the muscular system.

1. The short extensor of the thumb ordinarily affects the first phalanx.
2. As the movement increases in force the whole thumb is pulled back and the wrist muscles are innervated to prevent the movement from spreading to the wrist.
3. The short flexor of the thumb flexes the nail joint and helps to move the entire thumb toward the little finger.
4. The chief abductor extends the nail joint, moves the middle phalanx sideways, and abducts the entire thumb.
5. A combination of these functions (although opposite in kind) are also found in the thumb-abductor.

The thumb muscles are a specialized group of muscles responsible for the thumb's incredible range of motion, strength, and dexterity. These muscles are essential for grasping, pinching, and manipulating objects, making the thumb a vital part of human hand function. Here's an overview of their anatomy, purpose, and function:
Anatomy of the Thumb Muscles
The muscles associated with the thumb can be categorized into two groups based on their location: a) intrinsic muscles (within the hand) and b) extrinsic muscles (originating from the forearm).

1. Intrinsic Thumb Muscles (Thenar Muscles) Located in the thenar eminence (the fleshy part of the palm near the thumb), these muscles are responsible for fine motor control and precision movements.
   • Abductor Pollicis Brevis:
     • Location: Superficial and lateral in the thenar eminence.
     • Function: Abducts the thumb (moves it away from the palm), enabling opposition and grasping.
   • Flexor Pollicis Brevis:
     • Location: Medial to the abductor pollicis brevis.
     • Function: Flexes the thumb at the carpometacarpal and metacarpophalangeal joints.
   • Opponens Pollicis:
     • Location: Deep to the abductor pollicis brevis.
     • Function: Allows the thumb to oppose the fingers by rotating and flexing the carpometacarpal joint.
   • Adductor Pollicis:
     • Location: In the web of the hand between the thumb and index finger.
     • Function: Adducts the thumb, pulling it toward the palm for a strong grip.
2. Extrinsic Thumb Muscles These muscles originate in the forearm and extend into the hand to control thumb movement. Here is the HTML unordered list with the child elements nested into another unordered list:
   • Flexor Pollicis Longus:
     • Location: Deep in the forearm, tendon runs to the thumb.
     • Function: Flexes the thumb, particularly at the interphalangeal joint.
   • Extensor Pollicis Longus:
     • Location: Posterior forearm.
     • Function: Extends the thumb at the interphalangeal and metacarpophalangeal joints.
   • Extensor Pollicis Brevis:
     • Location: Adjacent to the extensor pollicis longus.
     • Function: Extends the thumb at the metacarpophalangeal joint.
   • Abductor Pollicis Longus:
     • Location: Posterior forearm.
     • Function: Abducts the thumb and assists in extension.

Purpose of the Thumb Muscles
The thumb muscles serve to:

• Provide the thumb with an unparalleled range of motion, including opposition, flexion, extension, abduction, and adduction.
• Facilitate precision grip (e.g., pinching) and power grip (e.g., grasping).
• Support fine motor tasks, such as writing, buttoning, and using tools.

Functions of the Thumb Muscles
The combined actions of the thumb muscles allow for:

1. Opposition:
   • The thumb moves across the palm to touch the fingertips, enabling grasping and manipulating objects.
2. Pinch Strength:
   • Movements such as holding a pen or picking up small objects rely on the coordination of the intrinsic and extrinsic thumb muscles.
3. Grasping Power:
   • Adduction and flexion of the thumb enhance grip strength for larger or heavier objects.
4. Stabilization:
   • Muscles like the adductor pollicis stabilize the thumb during activities requiring sustained pressure.
5. Dexterity:
   • The thenar muscles enable intricate movements needed for tasks like typing, playing musical instruments, and sewing.

Clinical Relevance

• Injuries or conditions such as carpal tunnel syndrome, De Quervain's tenosynovitis, or arthritis can impair thumb muscle function, leading to reduced grip strength and dexterity.
• Rehabilitation and strengthening exercises often focus on these muscles to restore function after injury.

By orchestrating complex and coordinated movements, the thumb muscles are central to the hand's versatility and utility in daily life.

The attachment of the muscular tendon to the rest-end is called the origin of the muscle. The tendonous attachment to the moveable bone is called the insertion. Between the origin and insertion of a muscle at least one articulation must intervene. In the case of several muscles, more than one joint intervenes. The entire problem of muscular action depends upon the relationship between muscle position and joint. The question as to which end of a muscle is its origin and which is its insertion cannot always be definitely answered until the movement is seen. Muscular contraction through the principle of the opposite equality of forces will exert a pull on both bones to which the muscle is attached. The bone offering the less resistance will be the one to move. Origin and insertion may change places. The origin of a muscle will be the end closer to the trunk. ^[2]

[1]thenar eminence: The thenar eminence ("palm of the hand") and the Latin word "eminentia", meaning projection) refers to the group of muscles on the palm of the human hand at the base of the thumb.

[2]longus pollicis: a muscle that arises dorsolaterally from the middle part of the ulna, extends the second phalanx of the thumb, and abducts the hand.

[3]trunk: n. [ME. tronke < OFr. tronc < L. truncus, a stem, trunk < truncus, maimed, mutilated < IE. *tronkus < base *trenk-, to press together, crowd, whence THRONG]
the main body or stem of a nerve, blood vessel, muscle, as distinguished from its branches.